In the present: Don't tell the person what you said.
First off, your friends should be well aware that if the other person finds out about what you said, you'll have an unpleasant situation to deal with. In high school, there were times when my friends and I made non-complimentary comments to each other about another student who had behaved obnoxiously. We trusted each other to not make our feelings known to others. We're friends; we're not going to set each other up for embarrassing social situations.
Friendships involve trust - the trust to understand another person's feelings and thoughts, and empathize when possible. That trust is often implicit; you shouldn't have to state outright that you don't want your friends telling this other person how they feel. If your friends share your secret, they've broken that trust. You would be within your rights to be angry at your friends - and they should know that. Backstabbing is not conducive to relationships.
I would not recommend telling the person what you said. Consider how you would feel if someone walked up to you and said, "Hey, I don't like you because [X, Y, Z]." You'd be offended even if they added "Oh, and I'm just doing this because my friends might tell you even though we were trying to safely talk about it behind your back."
Sure, your phrasing might be more tactful than this . . . but the sentiment is exactly the same. I highly doubt the person will respond better to you telling them to their face.
In the future: Keep these secrets truly a secret!
There are secrets I have - feelings, thoughts, desires - that I have never voiced to my friends. It's entirely possible to let a secret slip by accident, even in the cases of the best-intentioned people. Maybe a friend of yours is talking to another friend about the person you dislike, and they happen to mention that you, too, dislike the person.
My deepest secrets are known only to very close friends - or none at all. Accidents happen, people mess up, and there's no better way to avoid this than by keeping things inside. In the future, if you're worried about your friends keeping a specific secret, then simply don't tell them.